I know, we all say  "someday" about  this or that  we might say it about things we are dreaming of but feel are far off.   But do you say it about getting fit ?... "someday I will get back into a healthy routine" ...or..."Someday I will start working out again"  ?.. 

That is achievable and does not have to be far off in the future.  Making a plan to start and making a schedule to workout  could be what is holding you back and making you say "someday".   It's just a matter of making the time  .  Is that the case with you?  You WANT to but you feel you have to wait until the dust settles in your life to carve out some time?.. I understand that !   You have a lot to fit into your day and scheduling workout time may never make it on the calendar, plus where do you begin?..  

Keep it simple, begin with just a few minutes a day.  

And as for when -  Chances are the weekdays are less disruptive than the weekends so start on a Monday. Then you have a good routine going all during the week and just stick with that.